1. Field of the Invention
The present invention relates to a radio frequency Microelectromechanical System (RF MEMS) switch. More particularly, the present invention relates to an RF MEMS switch and a method for fabricating the same, in which the RF MEMS device is down bent at a low voltage.
2. Description of the Related Art
Generally, an RF MEMS switch is used in various fields. For example, an RF MEMS device is used as a band selector, a multi-function switch, or a phase shifter in mobile products.
Various kinds of RF MEMS switches have been developed. Examples of an RF MEMS switch include an electrostatic RF MEMS switch based on electrostatic phenomenon and a piezoelectric RF MEMS switch based on piezoelectric effect. FIGS. 1 and 2 respectively show these RF MEMS switches.
FIG. 1 is a front sectional view illustrating an electrostatic RF MEMS switch. Referring to FIG. 1, the electrostatic RF MEMS switch 10 includes a substrate 1 provided with an RF signal line 12a, an anchor 13, and a driving line 14, a cantilever 11 fixed to the anchor 13 at an interval of 1 μm from the RF signal line 12a, and a contact pad 12 formed at an end of the cantilever 11 to be switched on/off in contact with the RF signal line 12a in accordance with driving of the cantilever 11. If an external voltage is applied to the RF MEMS switch 10 through the driving line 14, electrostatic force occurs between the driving line 14 and the cantilever 11, so that the cantilever 11 is down driven to allow the contact pad 12 to transmit an RF signal in contact with the RF signal line 12a. However, the electrostatic RF MEMS switch 10 has a high driving voltage of 3V or greater and a high volume. Because of this, the general trend is to substitute the electrostatic RF MEMS device with a piezoelectric RF MEMS switch shown in FIG. 2.
FIG. 2 is a plan view illustrating a piezoelectric RF MEMS switch. Referring to FIG. 2, an RF MEMS switch 20 of lead zirconate titanate (PZT, Pb(Zr,Ti)O3), which is up driven, is shown. The piezoelectric RF MEMS switch 20 includes a substrate 1 plated with an RF input signal line 22a and an RF output signal line 22b, and a plurality of cantilevers 21a to 21d that support a contact pad 22 positioned below the RF signal lines 22a and 22b and spaced apart from the RF signal lines 22a and 22b. The cantilevers 21a to 21d comprise an upper electrode layer, a piezoelectric layer, a lower electrode layer, and a membrane. If a DC voltage is applied to the electrode layers of the cantilevers 21a to 21d through driving lines 24a and 24b, the cantilevers 21a to 21d are up bent in a cavity 23a. Then, the contact pad 22 formed at an end of the cantilevers 21a to 21d contacts the RF signal lines 22a and 22b so that the RF signal lines 22a and 22b are connected with each other to transmit an RF signal. The piezoelectric RF MEMS switch 20 can be driven at a voltage less than 3V, generates displacement of about 1.8 μm when the cantilever has a length of 100 μm, and has little power consumption.
However, there occurs some difficulty in fabricating the aforementioned piezoelectric RF MEMS switch 20. Particularly, the fabricating process of the piezoelectric RF MEMS switch 20 is not simple. In the piezoelectric RF MEMS switch 20, the piezoelectric layer or the membrane of the cantilevers is fabricated at a high temperature. For this reason, the piezoelectric layer or the membrane should be formed prior to a coplanar waveguide (CPW) line including the RF signal lines. If the CPW line is formed on the substrate and a piezoelectric thin film material is fabricated on the CPW line, diffusion of metal occurs at a high temperature or silicide is formed. Therefore, in the piezoelectric RF MEMS switch, as shown in FIG. 2, the cantilevers 21a to 21d are up bent and a separate wafer or substrate 1 is prepared on the cantilevers 21a to 21d so as to form the CPW line. In this case, a rear surface (bottom) of the substrate is excessively etched. In the piezoelectric RF MEMS switch 20 shown in FIG. 2, after the RF signal lines 22a and 22b are formed on the upper surface of the substrate 1 through plating, an opposite surface of the substrate 1 is fully etched so as to form the cantilevers 21a to 21d. 
To solve the difficulty in fabricating the RF MEMS switch, Korean Laid-Open Patent Nos. 2005-86629 and 2005-0076149 disclose a piezoelectric RF MEMS switch in which cantilevers are formed on a cavity so that they can be down driven. However, this piezoelectric RF MEMS switch separately requires a substrate provided with cantilevers and a substrate provided with an RF signal line. In this respect, if a CPW line and cantilevers are provided on one substrate in the piezoelectric RF MEMS switch, it is possible to provide a simple fabricating process of the piezoelectric RF MEMS switch. In such case, it is easy to form the CPW line, and switching operation of the RF MEMS switch would exactly be performed.